What is a yellow card in soccer?
A yellow card is an official warning for a foul or unsporting behavior. A second yellow in the same match becomes a red — you're sent off, and your team plays a man short.
Explain further
The yellow card is the referee's formal way of saying "knock it off." The referee holds a small yellow card overhead so everyone in the stadium knows a player has been officially "booked" (the name comes from the referee jotting the player's name in a little notebook). You can earn one for a reckless tackle, a handball that stops a promising attack, time-wasting, dissent toward the referee, or persistent fouling. It is a warning with teeth, because it stays on your record for the rest of the match.
The key thing for Americans to grasp is the math. A yellow is not a "free" foul like a personal foul in basketball, where you can rack up several before fouling out. A second yellow in the same game equals a red card, and that player is ejected with no substitute allowed. The team finishes the match with ten players instead of eleven, a real handicap that often decides the result.
A common point of confusion is that yellow cards also carry over across games in many tournaments and leagues. Collect a set number over a stretch of matches, often five, and you serve a one-game suspension even though you were never sent off. That is why you sometimes see a star player sitting out a big game for an offense that, on its own, looked pretty minor.